Firefox should create a new sessionstore.js file every time you start up, so it's very unusual not to have one. Is it possible you aren't looking in the right folder?

To access your current Firefox profile folder without opening Firefox and potentially overwriting your existing file, you can type or paste the following into the Run dialog (hold down the Window key and tap R, or use the second home screen which has all the applications, or use the search box on the charms bar) and press Enter to run it:

%APPDATA%\Mozilla\Firefox\Profiles

Usually you will just have one folder here to click into but if you have more than one, try the most recently updated one first. Can you find any sessionstore files?

If you think you deleted it, try opening the Recycle Bin and restoring the old file. Firefox should be closed when you do that, or you may crash it.

Find anything?

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)

• http://kb.mozillazine.org/Profile_folder_-_Firefox

The only session file I have is; sessionCheckpoints.json. My old sessionstore file is not in the Recycle Bin because the Bin was emptied.

I suspect that file is gone and unrecoverable. However, you could try an undelete program such as the following and see whether it discovers any sessionstore.js or sessionstore.bak files.

http://www.piriform.com/recuva

Undeleting doesn't have a great track record for finding intact/undamaged files, especially as time passes and Windows overwrites the old data, but sessionstore files list the URLs in readable form, so even a damaged file might be useful.

After that, could you check for customized session history settings in Firefox's about:config preferences editor. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste sess and pause while the list is filtered

(3) Review the preferences starting with:

• browser.sessionhistory.
• browser.sessionstore.

If you see bolded entries, which indicates that they vary from the default setting, could you right-click > Copy and post in a reply to see whether they might be relevant to this problem?

Okay, I have the sessionstore file back. Got it by using Recuva. The last modified date is 4/11/2014. Pinned tabs still are not opening after closing Firefox and reopening.

There are no bold, browser.sessionhistory, browser.sessionstore entries in about:config.

I actually do not use pinned tabs and am not familiar with how they work. However, I think all tabs should be stored/restored together, so if other tabs are restored and not the pinned tabs, that definitely seems odd to me.